Due to the ever-increasing preservation and reduced availability for logging of old-growth forests, more and more smaller diameter and lower quality logs are being supplied to the forest products industry to process into lumber and veneer-type products. These smaller and lower quality logs yield lower quality lumber and veneer than those produced from higher quality logs. A major concern with these lower quality products is their poor dimensional stability when exposed to environmental changes during the initial drying process, shipping, installation and service.
Lower quality lumber and veneer not only possess lower load-carrying capacities, but also contain more geometric defects such as cup, bow, crook and twist. Such geometric defects are especially critical when lumber and veneer, purchased at higher moisture contents, are installed in a structure built in a drier climate. The partially dried, lower quality lumber and veneer will continue to dry in place until they reach their equilibrium moisture content (EMC), resulting in additional warpage. Often, installed pieces warp to such an extent that they have to be replaced, resulting in considerable additional cost to an owner and to a construction company.
Up to this point, the primary focus of the major segment of the wood industry has been to produce a high-volume, low-cost product which can compete economically with steel and concrete. However, because of the ever-increasing construction labor cost, purchase of the cheapest product no longer ensures the most economical long-term construction cost. Dissatisfaction with the performance of lower quality wood products often turns the construction industry to the use of other, more expensive products, as witnessed by the rapidly increasing use of steel studs in commercial and industrial buildings.
While a segment of the wood industry is responding to these needs by producing highly engineered products, such as laminated veneer lumber and I-joists, these products still require high quality raw material for their production and do not utilize available lower quality logs, lumber or veneer.
Wood products, due to the biological origin of their constituents, possess complex mechanical and physical properties. The large variability associated with these properties further complicates the behavior of wood products. These complex behaviors are often represented by simplified mathematical models, which allow for a better understanding of the underlying principles and provide reasonable predictions of expected behavior.
For mechanical property representation, wood products are usually classified into anisotropic, orthotropic or isotropic behaviors. Anisotropy represents the most complex while isotropy the least complex behavior. Anisotropic behavior manifests itself in properties which vary with different directions in the materials.
Solid sawn lumber and veneer products generally represent anisotropic behavior due to the frequent presence of global and local cross-grain, the latter generally associated with knots. Anisotropic behavior manifests itself in shear coupling deformations caused either by externally-imposed stress, or by moisture or temperature-related deformation. Shear coupling causes twisting and cupping while bow and crook are associated with material inhomogeneity, i.e., uneven distribution of material quality within a piece of wood. An orthotropic material, on the other hand, does not possess the shear coupling phenomenon.
The second major factor is associated with the location of cross-grain due to global or knot-associated grain deviation at the edges of lumber and veneer. As wood dries or is loaded by a force causing significant stresses, cracks develop at the edges containing cross-grains. These cracks propagate along the grain direction, causing local separation and eventual failure of lumber or veneer.
Others have proposed various types of laminated wood products in efforts to upgrade low quality lumber. See, for example, U.S. Pat. Nos. 3,961,654; 4,012,548; 1,778,333; 2,039,398; 2,382,208; 4,776,919; 4,262,717; 598,437; 2,573,465; and 4,314,871.
None of the above previous art encompasses the fundamental concepts of the present invention of the mirror lamination.
U.S. Pat. No. 3,961,654 is aimed at increasing the yield from a round log when producing lumber and does not utilize rectangular laminae or mirror imaging.
U.S. Pat. No. 4,012,548 represents an attempt to produce symmetric laminations but it requires the use of four or more veneers. Veneers composing the faces of the lamination retain their parallel grain directions while the veneers for the inner two or more laminates are cut in such a way that the grains angle will be oriented within 10.degree. to 25.degree. from the face veneer grain angle. The primary objective of such patent is to improve machine workability relative to plywood and to provide better performance against cracking and chipping. While improved strength is cited, no specific strength properties are identified or reasons given why such improvements are to be derived. Dimensional stability of the resulting product is not addressed by the patent.
U.S. Pat. No. 1,778,333 relates to the manufacture of a lumber product with the primary objective of removing defects by cutting out the defective sections and re-gluing the remaining clear materials. This does not relate to the concept of mirror lamination.
U.S. Pat. No. 2,039,398 describes a method of producing a three-layer laminated beam by pre-stressing the outside laminae. When used in bending, this pre-stressing aids in the resistance of bending stresses and hence provides an increased load capacity. No mention of mirror imaging or dimensional stability has been cited in this patent.
U.S. Pat. No. 2,382,208 is aimed at gluing together different types of wood pieces and cutting curved surfaces into the laminate to produce ornamental structures. No matching of wood characteristics through mirror lamination is considered or suggested.
U.S. Pat. No. 4,776,919 describes a laminated lumber press apparatus. No description of the present invention is made therein.
U.S. Pat. No. 4,262,717 is aimed at producing a larger usable rectangular size of balsa wood from a small log and hence has similar goals to U.S. Pat. No. 3,961,654. Since higher yield is its main objective, it does not relate to the current invention.
U.S. Pat. No. 598,437 describes an invention to produce decorative effects for flooring and ceiling products. It also cites durability, strength and avoidance of shrinkage, but such general claims need interpretations. Strength there is likely to mean hardness, i.e., resistance to indentation resulting from walking on the floor; this is somewhat synonymous with durability. Avoidance of shrinkage results from alignment of pieces such that the radial direction of the wood is oriented in the large plane of the floor or ceiling. Since shrinkage in the radial direction of wood is approximately one-half of that in the tangential direction, this alignment improves the shrinkage performance. These concepts do not suggest the present invention.
U.S. Pat. No. 2,573,465 requires the boards, strips or the like to be forcibly bent and glued together in their bent condition. The so-bonded laminate is then cut along its long axis and the two layers are displaced lengthwise relative to each other. The so-aligned boards are glued together forming a dimensionally stable laminate. The concepts embodied in such patent do not represent any significant relation to the present invention due to forcible bending, two layer edgewise gluing and longitudinal displacements.
U.S. Pat. No. 4,314,871 defines the machinery which produces a tongue and groove laminate. It does not relate to or describe the present invention.
A primary objective of the present invention is to provide a laminated wood product having improved strength and dimensional stability.
Another objective of the present invention is to provide a laminated wood product which balances the internal forces thereof.
Another objective of the present invention is to provide value-added products which enhance the market opportunity of wood products through improved performance of lower grade lumber and veneer products.
The objectives of this invention and associated benefits will become apparent from the following detailed description and accompanying drawings.